1. Field of the Invention
The present invention relates to an input protection circuit for a semiconductor integrated circuit, and, more particularly, to an input protection circuit which protects a semiconductor integrated circuit against a static damage.
2. Description of the Related Art
There has been a known problem that when static electricity of a high voltage is applied to a semiconductor integrated circuit (hereinafter referred to as “IC”), the internal circuit of the IC is statically damaged. To prevent such a static damage on an IC, various techniques have been proposed.
As an ordinary technique to prevent a static damage on an IC, to protect the internal circuit of the IC (hereinafter also referred to as “IC internal circuit”) against static noise or the like, a plurality of static protection diodes are arranged in series between both power supplies at the peripheral portion of an input terminal to which a signal is input. The static protection diodes are connected between the input terminal and the positive power supply and between the input terminal and the negative power supply. Therefore, it is not possible to input a voltage signal with a voltage (negative power supply voltage−diode's threshold voltage) and a voltage signal with a voltage (positive power supply voltage+diode's threshold voltage) to the input terminal.
Such a conventional input protection circuit will be discussed below referring to FIGS. 1 to 4. FIG. 1 is a block diagram showing one example of the conventional input protection circuit. As shown in FIG. 1, the input protection circuit is constructed by connecting protection diodes D1001 and D1002 between an input terminal T1001 and a positive power-supply terminal T1002 and between the input terminal T1001 and a negative power-supply terminal T1003, respectively. In the conventional input protection circuit shown in FIG. 1, the protection diodes D1001 and D1002 are connected directly to the input terminal T1001. An input protection circuit which is constructed like the circuit in FIG. 1 is disclosed in, for example, Japanese Patent No. 2757701 and Japanese Patent Laid-Open No. 2000-22077.
Given that VDD is a positive power-supply potential level, VSS is a negative power-supply potential level and VF is a threshold voltage of the protection diodes D1001 and D1002 (also called a forward drop voltage which is generally about 0.7 V), when a potential V1 to be applied to the input terminal T1001 becomes greater than VDD+VF, the protection diode D1001 is turned on. When the potential V1 to be applied to the input terminal T1001 becomes smaller than VSS−VF, on the other hand, the protection diode D1002 is turned on. As shown in FIG. 2, therefore, the conventional input protection circuit shown in FIG. 1 cannot transfer the input signal exceeding VDD+VF or the input signal lower than VSS−VF to an internal circuit 1001.
Even if the potential of the input signal to be input to the input terminal T1001 does not become greater than VDD+VF, the forward bias voltage is applied to the protection diode D1001 when the potential of the input signal exceeds VDD. This produces a forward leak current I1002 from the input terminal T1001 toward the positive power-supply terminal T1002.
Further, in case where the input impedance of the internal circuit 1001 is finite, when part of an input signal current I1001 that should inherently flow toward the internal circuit 1001 flows toward the positive power-supply terminal T1002 via the protection diode D1001 when the input signal potential V1 exceeds VDD, as shown in FIG. 3. As a result, the input signal waveform is distorted by the leak current I1002 that flows, separated, toward the positive power-supply terminal T1002 from the protection diode D1001.
The same is true of the protection diode D1002, and a forward leak current I1004 is produced in the protection diode D1002, as shown in FIG. 4, when the input signal potential gets lower than VSS, thus deforming the input signal waveform.
In case of the input protection circuit disclosed in Japanese Patent Laid-Open No. 2000-22077 where elements, such as input resistors, of an inverting amplifier circuit are connected between the input terminal T1001 and the protection diodes D1001 and D1002, when static electricity of a high voltage which statically damages the internal circuit of the IC is applied, the elements to be connected to the input terminal T1001 are statically damaged before the internal circuit of the IC is statically damaged.
As discussed above, the conventional input protection circuits had a problem such that an input signal whose potential would exceed the positive power-supply potential or an input signal whose potential would become lower than the positive power-supply potential would be deformed. Further, in case where an attempt is made to solve the problem by the method disclosed in Japanese Patent Laid-Open No. 2000-22077, the input resistor may be statically damaged.